This invention relates to the manufacture of draw tape plastic bags and more particularly to making a hem in the bags for holding the tape.
Bags made of thin polyethylene material have been used in various sizes. Small bags are used in the packaging of sandwiches and the like; larger bags are used as shopping bags; and even larger bags are used for containing trash.
A particularly advantageous closue for such a bag includes a draw tape constructed from the same polyethylene material. U.S. Pat. No. 3,029,853--Piazzi, and British Pat. No. 1,125,363--Jortikka are examples of draw tape bags. Such closures have been successfully employed on these bags.
Draw tape closures for large trash bags, and the manufacture of these draw tape trash bags, are described in the related applications identified below.
Forming the hem, into which the draw tap is inserted, is shown, for example, in U.S. Pat. Nos. 2,897,729--Ashton et al, 3,058,402--Kugler and in METHOD AND APPARATUS FOR FORMING HEMS IN SUPERPOSED PLIABLE PANELS, Boyd et al, Ser. No. 652,255, filed Sept. 20, 1984, now U.S. Pat. No. 4,617,008.
One of the problems encountered in prior art hem forming apparatus is brought about by sharp turns in the direction of the moving web of film which forms the bag. It is desirable to move this web smoothly in one direction without sharp turns, even during the hem forming operation. The sharp turns create drag which makes it difficult to control the tension in the web.
Polyethylene is extensible to some extent, which makes it easier to process and form than stiffer materials such as polystyrene, paper or cellophane. Nevertheless, forming apparatus for polyethylene must move the web smoothly and without distortion. Any movement that requires one portion of the moving web to travel a longer path than another introduces distortion.
It is an object of the present invention to fold a hem in a moving web of film without drastically altering the direction of movement of the web.
It is another object of the present invention to form hems in the opposing panels of a moving web of folded thermoplastic film in the manufacture of draw tape bags.